1. Field of the Invention
This invention relates to heat-sensitive recording materials and more particularly to fixable heat-sensitive recording materials of the diazo type.
2. Description of the Prior Art
Heat-sensitive recording materials are well known which are adapted to produce record images by thermally contacting a colorless basic dye with a color developing material to utilize color-forming reaction occurring on contact of the color developing material with the dye.
These conventional heat-sensitive recording materials are designed to produce record images by being heated and therefore have the following drawback. Even after record images have been formed by application of heat, color-forming reaction still takes place when they are inadvertently heated. As a result, the record images will become obscure or illegible owing to the coloration of the background portion thus heated. For this reason, the heat-sensitive recording materials of this type are unfit for use in recording important information to be stored.
In recent years, research is under way to develop fixable heat-sensitive recording materials of the diazo type utilizing color-forming reaction between a diazonium salt and a coupler.
Generally heat-sensitive recording materials of the diazo type include a recording layer formed on a substrate and having dispersed therein particles of a diazonium salt, a coupler and a color developing auxiliary capable of producing a base by being heated. On heating the recording layer of this type of heat-sensitive recording material, the color developing auxiliary produces a base which causes color-forming reaction (diazo coupling reaction) of the diazonium salt with the coupler to give record images. Thereafter the entire surface of the recording layer is irradiated with ultraviolet rays to decompose the unreacted diazonium salt in the unrecorded portion of the recording layer. The decomposition of the unreacted diazonium salt eliminates the possibility of color-forming reaction occurring on application of heat, whereby the record images are fixed.
Known color developing auxiliaries capable of producing a base when heated include substances producing a base on thermal decomposition such as organic or inorganic ammonium salts, urea and the like. However, since these substances gradually decompose even at ambient temperature, diazo coupling reaction between the diazonium salt and the coupler is caused during the storage of the recording material (this reaction will be hereinafter referred to as "precoupling"), thereby giving rise to undesired coloration (fogging) of the recording layer.
It has been also proposed to use, as such color developing auxiliary, a substance capable of producing a basic atmosphere by being thermally fused, such as guanidine derivatives and aliphatic amines, e.g., stearyl amine. The use of these substances, however, results also in occurence of precoupling during storage especially under humid conditions and consequently in coloration (fogging) of the recording layer.
British Patent No. 1,128,762 also discloses various heat sensitive organic bases, which include amidines, in particular, N,N'-diphenylformamidine and benzamidines in which one of the two nitrogen atoms may optionally have a substituent selected from the group consisting of benzyl, cyclohexyl, phenyl and 2-pyridyl, such as benzamidine, p-toluylamidine, N-benzylanisamidine, N-cyclohexylanisamidine, N-phenylbenzamidine, N-(2-pyridyl)benzamidine and the like and N-phenylacetamidine. According to our investigation, however, when N,N'-diphenylformamidine and similar formamidines are used, the resulting recording materials have a relatively good storage stability, but when stored for a prolonged period of time, e.g. for about 6 months, under normal condition, the recording materials give rise to a marked fogging due to the precoupling and emit strong disagreeable amine-like odor. Similarly the use of the foregoing benzamidines or N-phenylacetamidine also results in precoupling and often causes strong amine-like odor to be emitted during storage for a prolonged period of time, e.g. for about 6 months, under normal condition.
U.S. Patent No. 3,389,995 discloses as the heat-sensitive color developing auxiliary an amidine compound of the formula ##STR3## wherein X is methyl, ethyl, propyl and the like.
According to our investigation, these amidines were also found to cause precoupling or emission of disagreeable amine-like odor when the recording material is stored for a long time, e.g., for about 6 months, under normal condition.